Implantable cardioverter defibrillators (ICDs) are used to provide various types of therapy to treat cardiac arrhythmias in a patient, including, for example defibrillation. These devices typically consist of a hermetic housing implanted into a patient and connected to at least one defibrillation electrode. The housing of the ICD contains electronic circuitry for monitoring the condition of the patient's heart, usually through sensing electrodes, and also contains the battery, high voltage circuitry and control circuitry to generate, control and deliver defibrillation shocks. Typically, one or more of the defibrillation electrodes are connected to circuitry within the ICD via one or more implantable cardiac leads that extend from the housing to the defibrillation electrodes. The housing of the ICD (usually referred to as the “CAN”) may also include one or more defibrillation electrodes configured on the exterior of the housing.
Implantable transvenous ICD leads are generally elongated lead bodies made of biocompatible insulation material(s) including multiple parallel lumens with each lumen carrying one or more conductors that run between connectors on a proximal end to electrodes proximate a distal portion of the implantable cardiac lead. The number of conductors required for true-bipolar ICD implantable cardiac leads is typically four (two conductors for sensing and pacing that provide conduction paths for a lower power sensed signal and a ground return, and two conductors for therapy that provide conduction paths for higher power defibrillation shocks). Integrated-bipolar leads can combine 1 defibrillation electrode as a pace-sense electrode and thus have only three conductors. In addition, a separate center inner coil and stylet lumen may be provided for use in implanting the ICD implantable cardiac lead. The center inner coils may also include conductors that carry electric signals to pacing sense/therapy electrodes. The diameter of the implantable cardiac lead body must be small enough to navigate the blood vessels through which the implantable cardiac lead is implanted, while still being robust enough to maintain electrical and mechanical integrity over the course of bending and movement during hundreds or thousands of heart beats and respirations.
The long-term reliability and safety of implantable cardiac leads is a significant issue. Conductor anomalies in the implantable cardiac leads for ICDs can result in morbidity or mortality from loss of pacing, inappropriate ICD shocks, or ineffective treatment of ventricular tachycardia or ventricular fibrillation. The early diagnosis of conductor anomalies for implantable cardiac leads is a critically important step in reducing these issues and making ICDs safer.
A particular conductor anomaly that is unique to implantable cardiac leads occurs when a conductor migrates through the soft silicone material of the implantable cardiac lead body away from the original position of the conductor within a lumen. In some cases, the cable including the conductor may abrade against the lumen that constrains it to migrate outwardly within the silicone implantable cardiac lead body (“inside-out” abrasion) without breaking through the external insulating layer of the implantable cardiac lead body. In other cases, the conductor may continue to abrade against the silicone elastomer lead body until it breaks through the surface and become externalized and exposed to body fluids and tissue. At this stage, it may be detected by fluoroscopy. Initially, the thin polymer (ETFE) insulating layer surrounding the cable remains intact, at least without delivery of high-voltage shocks. Over time, this ETFE secondary insulating layer can become abraded or damaged due one of various mechanisms. Exposed conductors can also result in sensing of nonphysiological electrical signals, “noise”, if the exposed conductor is connected to a sensing electrode. This results in incorrect detection of ventricular tachycardia or fibrillation (over-sensing) that may result in unnecessary painful shocks. Even more worrisome, this problem may result in failed defibrillation shocks if the conductor is connected to the primary (distal) shock coil located in the right ventricle (RV). In this case, the patient will likely die of the arrhythmia unless promptly defibrillated by an external defibrillator. Failed defibrillation shocks are particularly likely if the conductor to the RV shock coil abrades against the proximal shock coil in the superior vena cava (SVC) resulting in a short circuit when a shock is needed. Such “under-the-coil” abrasions may occur without exteriorized cables so that they are undetectable by fluoroscopy, hence only detected when a shock is delivered. It is not known how often or the extent to which early-stage failures of outer insulation may compromise sensing and/or defibrillation.
Some leads have an additional, abrasion-resistant coating of silicone-polyurethane copolymer (e.g. Optim™) tubing on the external surface. Intact, external tubing prevents cables that have abraded through their lumens from exteriorizing, but it does not alter the fundamental process of “inside-out” abrasion. Under the shock-coils, coated leads are identical to similarly-designed leads without tubing, and they provide no additional protection against inside-out, cable-coil abrasion. It is desirable to detect inside-out abrasions in coated leads even when an electrical abnormality resulting in lead failure does not occur.
Numerous approaches have been suggested for trying to diagnosis and correct for the problems of implantable cardiac lead failures and anomalies. Most involve the classic approach of subjecting the implantable cardiac lead to a periodic test pulse, measuring the direct-current impedance of the test pulse and then comparing that impedance to an expected range of acceptable impedance values. In implantable cardiac leads, however, only one end of the conductor is generally accessible for testing purposes and changes in system impedance may be dominated by changes unrelated to conductor or implantable cardiac lead faults. For example, the reference impedance for pace-sense conductors (the metal only) is in the range of about 15-50Ω, and usually is constant within about 10% for an individual conductor. But the reference impedance for the combined electrode-tissue interface and connected body tissue ranges from about 300Ω to greater than 1000Ω. More importantly, biological variations of up to 300Ω are common, and variations of greater than 1000Ω may occur without conductor or insulation failures.
U.S. Pat. Nos. 8,700,156, and 8,352,033 (the disclosures of which are incorporated by reference herein) to Kroll teach an ICD circuit for discharging a small capacitor for lead testing. However, that approach has a significant implementation complication that renders it unattractive as it requires the addition of high-energy switches to isolate the main high-energy therapeutic capacitors during testing.
A very critical problem presents when the insulation is sufficient to withstand low-voltage test pulses and is thus deemed to be intact. If the patient needs high-voltage therapy then a defibrillation shock of typically 800 or 900 V is delivered. This high-voltage shock can arc through the thin insulation and result in a catastrophic failure.
A need therefore exists for methods and apparatus that can analyze and identify implantable cardiac lead conductor anomalies at the subclinical stage, before they present as a clinical problem, and do so with a high sensitivity and specificity that minimizes false positives for implantable cardiac lead conductor anomalies.